Strange Pressure Testing Results
Strange Pressure Testing Results
(OP)
Hi all. Not sure if this is the right group.Here it goes:
Im currently involved in pressuretesting of a repairclamp which is mounted over a pipe. During pressuretesting with water at the volume between the clamp and pipe, there is a continous small pressure drop over time but no visible leak... The water volume is 30liters and it is pressurized to 250Bar. After 4 days of testing the leak rate is approx 0.1ml/h. Since there is no visible leaks a hydrogen mix was induced into the water and a sniffer was used to see if there was any leaks. The sniffer didnt catch any leaks! (even at 400Bar!)
So whats causing this pressure drop ?. Are we looking at steel relaxation or continous compression of the water over time. Have anyone experienced such phenomenas before ? Is this a "creep" thing.
All speculations is appriciated since im currently knocking my head against the wall trying to find an explenation... :)
Brgds Odd Einar
Im currently involved in pressuretesting of a repairclamp which is mounted over a pipe. During pressuretesting with water at the volume between the clamp and pipe, there is a continous small pressure drop over time but no visible leak... The water volume is 30liters and it is pressurized to 250Bar. After 4 days of testing the leak rate is approx 0.1ml/h. Since there is no visible leaks a hydrogen mix was induced into the water and a sniffer was used to see if there was any leaks. The sniffer didnt catch any leaks! (even at 400Bar!)
So whats causing this pressure drop ?. Are we looking at steel relaxation or continous compression of the water over time. Have anyone experienced such phenomenas before ? Is this a "creep" thing.
All speculations is appriciated since im currently knocking my head against the wall trying to find an explenation... :)
Brgds Odd Einar





RE: Strange Pressure Testing Results
RE: Strange Pressure Testing Results
The three things that come to mind are:
1. How are you making certain that the water temperature is absolutely constant (temp change of 1F is about 100 psi)?
2. How are you making sure that the test water is absolutely free of disolved gases (with time even at 400 bar some disolved gas can evolve and give you strange results)?
3. Finally, how are you making certain that you aren't leaking down the pipe (are you testing against a valve or stopple, or using an insert or bolt-on blind)?
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Strange Pressure Testing Results
RE: Strange Pressure Testing Results
The results are varying a lot with workshop temperature and all temperatures are logged over 4 days to give a trend curve which shows a slowly dropping pressure.
"ZDAS04" - The clamp is 800mm long mounted over a pipe with both radial and longitudinal seals against the test pipe in both clamp ends.
RE: Strange Pressure Testing Results
A drop of 0.1oC over 4 days could justify such a volume ("leak") contraction rate.
RE: Strange Pressure Testing Results
SWAG's
Where are you located?
Under the right conditions a very small leak/leaks can evaporate.
How are the two halves of the clamps sealed, IE mechanical or welded?
Did you use H2 or He?
Small amounts of H2 are hard to detect.
What type pressure gauge are you using?
RE: Strange Pressure Testing Results
RE: Strange Pressure Testing Results
RE: Strange Pressure Testing Results
It would be nice to see a continuous plot of pressue and temperature through the length of the test.
http://virtualpipeline.spaces.msn.com
RE: Strange Pressure Testing Results
Thanks all. At least some useful experiences were achieved :)